1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of optimizing the image forming conditions.
2. Related Background Art
Electrophotographic copying machine is already well known as an image forming apparatus.
Such copying machine is equipped therein with a cylindrical photosensitive drum, bearing a photosensitive layer on the external periphery thereof, and said photosensitive drum is driven in a predetermined direction by driving means. Along the periphery of said photosensitive drum, there are provided, in succession along the direction of rotation, primary charging means, exposure means, developing means, image transfer means, cleaning means and charge eliminating means.
In such configuration, the external periphery of the photosensitive drum is charged to a predetermined potential by the primary charger, and thus charged portion is exposed by the exposure means along with the rotation of the photosensitive drum. The surface potential of the external periphery of the photosensitive drum varies according to the amount of exposure to light, whereby an electrostatic latent image is formed in continuation on said external periphery. Said electrostatic latent image is rendered visible by deposition of toner from the developing means, and the visible image thus formed is transferred, by the image transfer means, onto a sheet material. The toner remaining on the photosensitive drum is removed by the cleaning means, and the surface potential is then eliminated by the charge eliminating means.
The image forming apparatus explained above is susceptible to variations in the ambient conditions because of the use of an electrostatic process, and is associated with drawbacks of propensity to cause unevenness in density resulting from uneven charging and an insufficient density resulting from deterioration in the developing ability, and of insufficient durability. These drawbacks result from smear in the primary charger, deterioration of charging ability of the photosensitive drum, deterioration of charging ability of toner etc. For avoiding these drawbacks, there is already known a method of measuring the surface potential of the photosensitive drum with a potential meter and optimizing the amounts of charging and exposure according to the result of said measurement. Also there is known a method of measuring the visible image formed on the photosensitive drum with a reflective densitometer and thus optimizing the developing conditions and the concentration of developer.
However, since the measuring devices in such conventional technologies are fixed in a direction perpendicular to the rotating direction of the photosensitive drum, it is difficult to detect the state of latent or visible image over the entire photosensitive member.
Also in the conventional analog image forming methods, in which the photosensitive member is directly exposed to the light reflected or transmitted by the original image, it is difficult to correct local image defects even if the measurement is conducted in a direction perpendicular to the rotating direction, and such defects can only be made less conspicuous by improvement in the entire level of image.
Furthermore, since the measuring devices are fixed in a direction perpendicular to the rotating direction, there may result erroneous control due to such local detection, though the image optimization is possible on the position of measurement.
Furthermore, since the measurements of potential and reflective density have conventionally been conducted independently, there may be applied an excessive load on the object of control. For example, in case the image density is low while the potential of the latent image is at a target value, the restoration of image density solely by the developing conditions may result in background smear or a long time required for such density restoration.
In the following there will be given an explanation specifically on the corona charger. FIG. 38 is a schematic view of a copying machine, as an example of the image forming apparatus of the above-explained kind. A cylindrical photosensitive member 100 rotates in a direction X, about an unrepresented axis. Around the photosensitive member 100 and along the rotating direction thereof, there are provided, in succession, a corona charger 101 serving as discharge means, developing means 102, a transfer charger 103, a separating charger 104 and cleaning means 105.
In the above-explained configuration, after the surface of the photosensitive member 100 is charged by the corona charger 101, said surface is exposed to a light image Y by unrepresented exposure means, in order to form a latent image. Said latent image is rendered visible by the deposition of toner from the developing means 102, and said toner image is transferred by the transfer charger 103 onto a transfer sheet 106. Said transfer sheet 106 is separated from the photosensitive member 100 by the separating charger 104, while the photosensitive member 100 is subjected to the removal of the remaining toner by the cleaning means 105.
Subsequently the transfer sheet 106 is advanced to an unrepresented fixing unit, for fixing said visible image.
The corona charger 101 is smeared, in the course of repetition of the above-explained copying operation, by the deposition of floating toner generated from the developing means 102, paper dusts, vapor of silicone oil used in the fixing unit, substances produced in the discharge etc. When such contaminating substances are deposited on a discharge wire (extending in the axial direction of the photosensitive member 100), a grid, shields etc. constituting the corona charger 101, the discharge becomes uneven in the direction of said wire, thus resulting in deterioration of the obtained visible image, since such unevenness in discharge results in uneven charging on the photosensitive member 100, thus giving rise to density unevenness or streaks on the visible image. Such smear, when aggravated further, leads eventually to abnormal discharge, which may damage the photosensitive member. For suppressing such smear of the charger 101, there has already been provided an automatic cleaner (not shown), which, for example in case of cleaning the discharge wire, sandwiches said wire with a wiping member for example of felt or sponge and moves along the wire thereby removing the smearing substance from the wire.
However, since such smearing substance sticks firmly to the discharge wire, it cannot be eliminated completely by such cleaning operation and accumulates gradually on the wire after repeated discharges, so that the unevenness in discharge occurs sooner or later. Consequently, the unevenness in discharge can only be eliminated completely by the replacement of the discharge wire. On the other hand, since the developing rate of contamination is heavily dependent on the ambient conditions (temperature, humidity, amount of dust etc.) and the conditions of use (frequency of copying, kind of paper used etc.), it is extremely difficult to predict the number of copies when the unevenness in discharge occurs. For this reason, the discharge wire is replaced after the deterioration in image quality by the discharge unevenness appears, and this situation has resulted in the deterioration of quality of the apparatus.
Besides the photosensitive member has to be replaced if it is damaged by the abnormal discharge, and such replacement not only complicates the maintenance work but also elevates the running cost of the apparatus. Although the deterioration of image can be prevented by the wire replacement prior to the appearance of uneven discharge, such wire replacement has generally to be conducted amply before the occurrence of such uneven discharge because the timing of appearance of such uneven discharge is difficult to predict as explained above and also because the inspection of the apparatus by the servicing personnel cannot be very frequent. Therefore, such method results in an increased number of maintenances and a wasted consumption of discharge wires.